What strikes me is that your parents sound so loving and seem to have given you a good childhood and raised their family by the JW handbook,yet both of their children were put out of the organization for one reason or another. They must be wondering how this could be...where they went wrong. They must wonder why the children of their "worldly neighbors" and or non JW family members are all still together as a family while they are alone with nothing to show in their old age, for their years of hard work doing things the right way...train up a boy in the lord and he will not turn aside from it. So what happened ?? What would the rest of the community think of them if they knew how they were treating you? What would they do if you wrote a letter to the local newspaper honestly admitting your past errors and how you have asked for your parents forgiveness only to have them tell you that God is going to one day soon, kill you and everyone else who isn't a JW. Doesn't it sound like they are throwing you under the bus to save themselves?
Your Dad would probably spend an hour or two with a neighbor or former co-worker yet against every instinct he has, would turn his own son away without seeing him unless there was a family emergency. They do this at the word of a faceless organization who has no idea who they are or what they are doing.
My Grandfather in Europe was raised as a Plymouth Bretheren ( like Amish) he was shunned for the rest of his life for not marrying in the Lord as the bible says. He and my Grandmother who was a Protestant raised their 6 children in the same small village without any of them ever speaking again. The Plymouth Bretheren thought they were the only true religion and were carrying out God's will. Even so, my Grandfather and his Parents are dust now and they wasted their time together here on this earth over pride and needing to be right or at least save themselves from the wrath of God. Your parents would think my Grandfathers family was wasting their time shunning their Son since what they belonged to was a false religion anyway, yet they don't realize they are doing the same thing.
What your Mother said about how association with you would weaken their faith, is quite telling. It seems that on some level JW's know that their beliefs, even with the aid of their almighty God's spirit behind them, can't hold up under the glare of any kind of honest scrutiny. They realize they are only a few unscripted comments away from chucking the whole thing and it scares them. The only way for them to survive in the light of day is to surround themselves only with others who will feed the fantasy they hold.
I would say to them. Mom..Dad...none of us knows that when we walk out the front door each day, that we'll make it back safely. Ask your parents if they could, for your sake, just trust that Jehovah's spirit is strong enough to protect them from the bad influence they seem to think you are capable of when you come visit them once in awhile.